The present invention relates generally to fluorescent lamps having starter filaments and to the conservation of energy used in operation of such lamp filaments and lamps. More specifically it relates to means and method for eliminating the power to the lamp filaments once the lamp is in operation but also permitting rapid start after shutoff.
It is known that fluorescent lamps have filaments connected at each end of the lamp external contact members. These external contacts supply power through the filament and heat it during the operation of fluorescent lamps.
The heating of the filament helps the emission of electrons from the cathode and helps to initiate the formation of the fluorescent plasma within the lamp. However, once the gas within the fluorescent tube is ignited into a plasma the continued heating of the filament is not necessary. This continued heating of the filament consumes approximately 10% of the energy consumed by an ordinary fluorescent lamp of about 30 watts during its operation.
It has been previously conceived to provide a switch which can be located in and operate in the internal portion of conductive leads extending through the lamp envelope. Such conductive leads extend into the enclosure of the lamp and extend from the interior of the lamp envelope to opposite ends of the lamp filament. This internal switch opens and closes responsive to the level of heating of the filament. Once a filament has been heated by the electric current and the fluorescent plasma has been started, the discharge causes a continued heating of the filament so that a switch responsive to heat of the filament will remain open while the fluorescent plasma remains on within the envelope of the fluorescent tube. This is a well developed art and the original patent on thermally responsive switches in fluorescent lamps has now expired.
Bimetal metallic strips are used as the heat sensitive and heat responsive moveable element of the switch. However, there are several problems associated with the use of conventional bimetallic strips in a heat responsive switch type of application.
For fluorescent lamp filament current switching, the switch is required to open at about 200.degree. C. To achieve this thermal opening, a preset stress opposing the opening might conventionally be mechanically applied by bending the contact wire to bias the bimetal strip into a closed position pressing against a stationary contact wire.
However, storage of such lamps as in unheated warehouses will lead to thermally induced stresses in the bimetal strip which will greatly add to the preset stresses applied when applied as the switch is manufactured.
Similarly, the use of the fluorescent lamp out-of-doors in winter will lead to similar induced stresses. Further, the manufacture of the fluorescent lamp itself results in heating of the lamp and its parts to about 500.degree. C. This is the annealing temperature during manufacture. This heating also leads to the development of large stresses in the mechanism which is to serve as the switch during normal operation of the lamp.
The conventional and commercially available bimetallic strips have different patterns of deflection relative to temperature and some of these are plotted in FIG. 3 of the accompanying drawings. In this drawing, the material designations are those given by the manufacturer, Texas Instruments, and where the letter refers to the material with the higher coefficient of thermal expression and the number to the material of lower expansivity. It will be seen from the Figure that the sample B3, for example, undergoes very substantial deflection over the temperature range of 100.degree. below zero centigrade (i.e. -100.degree. C.) to 500.degree. or 600.degree. C. Very large deflections of such strips are accompanied by the imparting of very large stresses to the strips themselves. Such large stresses leads to concern regarding creep and fatigue failures.
Lower deflections and stresses can be obtained by employing a material such as that illustrated as E5. However, such materials having a shallower pattern of deflections relative to temperature have the disadvantage of moving slowly in the use temperature range of about 200.degree. C. and this leads to a different problem, namely "chatter". The chatter occurs when the opening and closing of the switch is the subject of less definite motion so that successive openings and closings (or chatter) occur as the temperature passes through the level at which the opening or closing will occur.
As an alternative, there is illustrated a material, N1, in FIG. 3 which does have a relatively flat pattern at the upper temperatures but which has substantially greater deflection even than the B3 material at the lower temperatures. Such large deflection results in large stresses and defeats the purpose of the switch that is in operation.
Reproducible operation at the desired temperature without interference from the high stresses developed either at high or low temperatures is desired. Referring here to FIG. 4, there is illustrated an idealized representation of what might be termed an ideal or perfect bimetallic strip and strip behavior for the switching application relative to the power saving for a fluorescent lamp. This ideal behavior is achieved by employing materials which display a differential coefficient of thermal expansion as follows: EQU .DELTA..alpha.=.alpha..sub.1 -.alpha..sub.2
where the value of .DELTA..alpha. is zero at all temperatures except at the intermediate temperature range where the opening of the switch is required.
.DELTA..alpha. is the differential coefficient of thermal expansion.
With a pattern such as that illustrated by the solid line in FIG. 4, i.e. a pattern in which there are no deflections outside of the use temperature, no stresses are produced outside this temperature region, either at high temperatures or at low temperatures.